Enhancements to Mobility Reference Signals for Radio Link Monitoring in a Beam-Based System

ABSTRACT

An access node transmits, in a downlink signal having a series of subframes, a beam-formed reference signal in subframes, where the beam-formed reference signals are transmitted in fewer than all of the subframes of the downlink signal. A first subset includes beam-formed reference signals corresponding to a first frequency or first localized range of frequencies, and a second subset includes beam-formed reference signals corresponding to a second frequency or second localized range of frequencies. The second frequency or second localized range of frequencies is spaced apart from and differing from the first frequency or first localized range of frequencies. A user equipment, UE, receives, in the downlink signal, the beam-formed reference signal in each of a plurality of subframes. The UE performs mobility management measurements using at least the first subset of the received beam-formed reference signals and performs RLM using the second subset of the received beam-formed reference signals.

TECHNICAL BACKGROUND

The present disclosure is generally related to wireless communicationssystems, and is more particularly related to access nodes that configurewireless devices to perform radio link monitoring (RLM) in such systems.

BACKGROUND

Radio Link Monitoring (RLM) in LTE

The Long-Term Evolution (LTE) wireless system developed by the3^(rd)-Generation Partnership Project (3GPP) is a widely deployedfourth-generation wireless communications system. In LTE and itspredecessor systems, the purpose of the RLM function in a wirelessdevice, referred to in 3GPP documentation as a “user equipment,” or“UE,” is to monitor the downlink radio link quality of the serving cellin RRC_CONNECTED mode. This monitoring is based on Cell-SpecificReference Signals (CRS), which are always associated to a given LTE celland are derived from the Physical Cell Identifier (PCI). RLM in turnenables the UE, when in RRC_CONNECTED mode, to determine whether it isin-sync or out-of-sync with respect to its serving cell, as described in3GPP TS 36.213, v14.0.0.

The UE's estimate of the downlink radio link quality, based on itsmeasurements of the CRS, is compared with out-of-sync and in-syncthresholds, Qout and Qin respectively, for the purposes of RLM. Thesethresholds are standardized in terms of the Block Error Rate (BLER) of ahypothetical Physical Downlink Control Channel (PDCCH) transmission fromthe serving cell. Specifically, Qout corresponds to a 10% BLER, whileQin corresponds to a 2% BLER. The same threshold levels are applicablewhether Discontinuous Reception (DRX) is in use or not.

The mapping between the CRS-based downlink quality and the hypotheticalPDCCH BLER is up to the UE implementation. However, the performance isverified by conformance tests defined for various environments, asdescribed in 3GPP TS 36.521-1, v14.0.0. Also, the downlink quality iscalculated based on the Reference Signal Receive Power (RSRP) of CRSover the whole band, as illustrated in FIG. 1, since PDCCH istransmitted over the whole band.

When no DRX is configured, out-of-sync occurs when the downlink radiolink quality estimated over the last 200-millisecond period becomesworse than the threshold Qout. Similarly, without DRX, the in-syncoccurs when the downlink radio link quality estimated over the last100-millisecond period becomes better than the threshold Qin. Upondetection of out-of-sync, the UE initiates the evaluation of in-sync.The occurrences of out-of-sync and in-sync are reported internally bythe UE's physical layer to its higher layers, which in turn may applylayer 3 (i.e., higher layer) filtering for the evaluation of Radio LinkFailure (RLF). The higher-layer RLM procedure is illustrated in FIG. 2.

When DRX is in use, the out-of-sync and in-sync evaluation periods areextended, to enable sufficient UE power saving, and depend upon theconfigured DRX cycle length. The UE starts in-sync evaluation wheneverout-of-sync occurs. Therefore, the same period (TEvaluate_Qout_DRX) isused for the evaluation of out-of-sync and in-sync. However, uponstarting the RLF timer (T310) until its expiry, the in-sync evaluationperiod is shortened to 100 milliseconds, which is the same as withoutDRX. If the timer T310 is stopped due to N311 consecutive in-syncindications, the UE performs in-sync evaluation according to the DRXbased period (TEvaluate_Qout_DRX).

The whole methodology used for RLM in LTE (i.e., measuring the CRS to“estimate” the PDCCH quality) relies on the assumption that the UE isconnected to an LTE cell, a single connectivity entity transmitting bothPDCCH and CRSs.

5G Development

In a study item for the new 5G radio access technology, entitled NewRadio (NR), companies have reached initial agreements on the followingdesign principles: ultra-lean design for NR; and massive usage ofbeamforming. Companies have expressed the view that beamforming shouldbe taken into account when RLM is designed, which is not the case inLTE. In addition, concerns have been expressed regarding how the UEshould measure the quality of a cell.

Following are some of the principles of NR that may drive the need fornew solutions for RLM, compared to the existing solution in LTE. Alsodescribed are some aspects of the beam-based mobility solution for NRusing RRC signaling across transmission receiving points (TRPs) that areunsynchronized and/or not sharing the same baseband and/or linked vianon-ideal backhaul.

Ultra-Lean Design in 5G NR

NR is expected to be an ultra-lean system, which implies a minimizationof always-on transmissions, aiming for an energy efficient future-proofsystem. Early agreements in 3GPP show that this principle has beenendorsed and there is a common understanding that NR should be a leansystem. In RAN1#84bis, RAN1 agreed, regarding ultra-lean design, that NRshall strive for maximizing the amount of time and frequency resourcesthat can be flexibly utilized or left blanked, without causing backwardcompatibility issues in the future. Blank resources can be used forfuture use. NR shall also strive for minimizing transmission ofalways-on signals and confining signals and channels for physical layerfunctionalities (signals, channels, signaling) within aconfigurable/allocable time/frequency resource.

Beamforming in 5G NR

There is a common understanding that NR will consider frequency rangesup to 100 GHz. In comparison to the current frequency bands allocated toLTE, some of the new bands will have much more challenging propagationproperties such as lower diffraction and higher outdoor/indoorpenetration losses. Consequently, signals will have less ability topropagate around corners and penetrate walls. In addition, in highfrequency bands, atmospheric/rain attenuation and higher body lossesrender the coverage of NR signals even spottier. Fortunately, operationin higher frequencies makes it possible to use smaller antenna elements,which enables antenna arrays with many antenna elements. Such antennaarrays facilitate beamforming, where multiple antenna elements are usedto form narrow beams and thereby compensate for the challengingpropagation properties. For these reasons, it is widely accepted that NRwill rely on beamforming to provide coverage, which means that NR isoften referred to as a beam-based system.

It is also known that different antenna architectures should besupported in NR: analog, hybrid and digital. This implies somelimitations in terms of how many directions can be coveredsimultaneously, especially in the case of analog/hybrid beamforming. Tofind a good beam direction at a given transmission point (TRP)/accessnode/antenna array, a beam-sweep procedure is typically employed. Atypical example of a beam-sweep procedure is that the node points a beamcontaining a synchronization signal and/or a beam identification signal,in each of several possible directions, one or few direction(s) at atime. This is illustrated in FIG. 3, where each of the illustrated lobesrepresents a beam, and where the beams may be transmitted consecutively,in a sweeping fashion, or at the same time, or in some combination. Ifthe same coverage properties apply to both a synchronization signal andbeam identification signal in each beam, the UE can not only synchronizeto a TRP but also gain the best beam knowledge at a given location.

As described above, common signals and channels in LTE are transmittedin an omnidirectional manner, i.e., without beamforming. In NR, with theavailability of many antennas at the base station and the different waysthey can be combined to beamform signals and channels, that assumption,as made in LTE, may no longer be valid. The major consequence of thatdesign principle of NR beamforming is that while in LTE it was quiteclear that the CRSs quality could be used to estimate the quality ofPDCCH, in NR this becomes unclear, due to the different ways channelsand reference signals can be beamformed. In other words, it cannot beassumed as a general matter that any particular reference signal will betransmitted in the same manner as the PDCCH is transmitted. Thisambiguity from the UE's point of view is due to the fact that referencesignals and channels can be transmitted by the network via differentkinds of beamforming schemes, which are typically determined based onreal-time network requirements. These requirements may include, forexample, different tolerance levels to radio overhead due to referencesignals versus control channels, or different coverage requirements forreference signals versus control channels.

Despite these challenges from NR design principles, an NR UE inconnected mode still needs to perform RLM, to verify whether its cellquality is still good enough, so that the UE can be reached by thenetwork. Otherwise, higher layers should be notified, and UE autonomousactions should be triggered.

Mobility Reference Signal in NR: 3GPP Agreements

In 3GPP discussions, certain aspects have been agreed to for mobilityreference signals (MRSs), which are used by the UE in NR formeasurements related to mobility (e.g., handover, or HO). Fordownlink-based mobility in RRC_CONNECTED mode involving radio resourcecontrol (RRC) and beams, the UE measures at least one or more individualbeams, and the gNB (3GPP terminology for an NR base station) should havemechanisms to consider those beams to perform HO. This is necessary atleast to trigger inter-gNB handovers and to avoid HO ping-pongs/HOfailures. It is to be determined whether UEs will report individualand/or combined quality of multiple beams. The UE should also be able todistinguish between the beams from its serving cell and beams fromnon-serving cells for Radio Resource Management (RRM) measurements inactive mobility. The UE should be able to determine whether a beam isfrom its serving cell. It is yet to be determined whetherserving/non-serving cell may be termed “serving/non-serving set ofbeams,” whether the UE is informed via dedicated signalling orimplicitly detected by the UE based on some broadcast signals, how thecell in connected relates to the cell in idle, and how to derive a cellquality based on measurements from individual beams.

Multiple solutions for the specific design of the MRS are beingconsidered, but in any of these, the UE performs RRM measurements withinits serving cell via a set of MRSs. The UE is aware of the specific MRSthat belongs to its serving cell, so that all other reference signalsthe UE may detect are assumed to be neighbors.

The transmission strategy for reference signals like MRSs can utilizethe freedom in time and/or frequency and/or the code/sequence dimension.By transmitting the reference signals for different beams in orthogonalresources, the network can obtain distinct measurement reportscorresponding to these signals from the UE corresponding to theorthogonal reference signals.

SUMMARY

As described above, RLM in LTE is based on CRSs, where a wide-bandsignal is transmitted in all subframes. A major consequence of thelean-design principle with respect to the RLM design in NR is that thereis a wish to avoid the design of wide-band signals transmitted in allsubframes. Therefore, lean design will prohibit the usage of the sameLTE solution for RLM in NR.

Described in detail below are techniques by which a wireless device(e.g., UE) can measure its serving cell quality where a cell istransmitting signals in a beamforming manner in a lean design, i.e.,without always-on reference signals transmitted in the whole band andacross all subframes.

According to some embodiments, a method in a user equipment, UE,includes receiving, in a downlink signal having a series of subframes, abeam-formed reference signal in each of a plurality of subframes, wherethe beam-formed reference signals are received in fewer than all of thesubframes of the downlink signal. The method also includes performingmobility management measurements using at least a first subset of thereceived beam-formed reference signals, the first subset correspondingto a first frequency or first localized range of frequencies. The methodfurther includes performing RLM using a second subset of the receivedbeam-formed reference signals. The second subset is at least partlydiffering from the first subset and includes beam-formed referencesignals corresponding to a second frequency or second localized range offrequencies. The second frequency or second localized range offrequencies is spaced apart from and differing from the first frequencyor first localized range of frequencies. The series of subframes of thedownlink signal may carry one or more control channels.

According to some embodiments, a method in an access node of a wirelesscommunications system includes transmitting, in a first downlink signalhaving a series of subframes carrying, a beam-formed reference signal ineach of a plurality of subframes, where the beam-formed referencesignals are transmitted in fewer than all of the subframes of thedownlink signal. The first subset includes beam-formed reference signalscorresponding to a first frequency or first localized range offrequencies, and the second subset includes beam-formed referencesignals corresponding to a second frequency or second localized range offrequencies. The second frequency or second localized range offrequencies is spaced apart from and differing from the first frequencyor first localized range of frequencies. The method also includesconfiguring a UE to perform mobility management measurements using atleast the first subset of the beam-formed reference signals and toperform RLM using at least the second subset of the beam-formedreference signals. In some embodiments, this configuring is performedprior to the transmitting. In some embodiments, the transmitting mayinclude transmitting a first control channel using the same beamformingparameters used to transmit the beam-formed reference signals.

According to some embodiments, a UE configured for operation in awireless communication network includes transceiver circuitry andprocessing circuitry operatively associated with the transceivercircuitry. The processing circuitry is configured to receive, in adownlink signal having a series of subframes, a beam-formed referencesignal in each of a plurality of subframes, where the beam-formedreference signals are received in fewer than all of the subframes of thedownlink signal. The processing circuitry is also configured to performmobility management measurements using at least a first subset of thereceived beam-formed reference signals, the first subset correspondingto a first frequency or first localized range of frequencies. Theprocessing circuitry is also configured to perform RLM using a secondsubset of the received beam-formed reference signals. The second subsetis at least partly differing from the first subset and includesbeam-formed reference signals corresponding to a second frequency orsecond localized range of frequencies. The second frequency or secondlocalized range of frequencies is spaced apart from and differing fromthe first frequency or first localized range of frequencies.

According to some embodiments, an access node of a wirelesscommunications system includes transceiver circuitry and processingcircuitry operatively associated with the transceiver circuitry. Theprocessing circuitry is configured to transmit, in a first downlinksignal having a series of subframes, a beam-formed reference signal ineach of a plurality of subframes, where the beam-formed referencesignals are transmitted in fewer than all of the subframes of thedownlink signal. The beam-formed reference signals include a firstsubset and an at least partly differing second subset, the first subsetincluding beam-formed reference signals corresponding to a firstfrequency or first localized range of frequencies, and the second subsetincluding beam-formed reference signals corresponding to a secondfrequency or second localized range of frequencies. The second frequencyor second localized range of frequencies is spaced apart from anddiffering from the first frequency or first localized range offrequencies. The processing circuit is configured to configure a UE toperform mobility management measurements using at least the first subsetof the beam-formed reference signals and to perform RLM using at leastthe second subset of the beam-formed reference signals. In someembodiments, the processing circuit is further configured to transmit afirst control channel using the same beamforming parameters used totransmit the beam-formed reference signals.

Further aspects of the present invention are directed to an apparatus,computer program products or computer readable storage mediumcorresponding to the methods summarized above and functionalimplementations of the above-summarized apparatus and UE.

Advantages of the embodiments disclosed herein may include, rather thancreating a large overhead by transmitting reference signals used formobility management (i.e., MRSs) over more frequency resources, the RLMperiodicity requirements can be larger than the mobility requirementssuch that replicated versions of the MRS are transmitted even moresparsely than the MRS, further reducing the overhead and/or the staticinterference. This can be switched off once there are no active UEs inthe cell. The network can also ensure that UEs can make more accurateRLM measurements over a large range of time-frequency resources, withoutintroducing a dedicated and static/always-on periodic RS in the network.

Additional advantages include that the signaling overhead can bemaintained at a low level, without compromising the accuracy of RLMmeasurements, especially during data inactivity. This can be a crucialrequirement in 5G NR. The network can also ensure that the RLM functioncan be maintained reliably for a control channel design without fallingback to a wider beam, as it can be essential to transmit the controlchannel on a narrow UE-specific beam for improved coverage at highcarrier frequencies.

Of course, the present invention is not limited to the above featuresand advantages. Those of ordinary skill in the art will recognizeadditional features and advantages upon reading the following detaileddescription, and upon viewing the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates how PDCCH can be scheduled anywhere over the wholedownlink transmission bandwidth.

FIG. 2 illustrates higher layer RLM procedures in LTE.

FIG. 3 illustrates a beam sweeping procedure.

FIG. 4 illustrates the generation of a single MRS.

FIG. 5 illustrates an MRS design in time and frequency domains.

FIG. 6 illustrates the principles of a reference signal transmissionthat facilitates RLM procedures described herein, according to someembodiments.

FIG. 7 is a block diagram of a network node, according to someembodiments.

FIG. 8 illustrates a method in the network node, according to someembodiments.

FIG. 9 is a block diagram of a UE, according to some embodiments.

FIG. 10 illustrates a method in the UE, according to some embodiments.

FIG. 11 is a diagram illustrating that RSs used for mobility can betransmitted on six adjacent PRBs in every fifth subframe, according tosome embodiments.

FIG. 12 is a diagram illustrating another example of how the MRSs may betransmitted, to support both mobility measurements and RLM, according tosome embodiments.

FIG. 13 is a diagram illustrating an example where the additional RSs atF2 and F3 are offset from one another, according to some embodiments.

FIG. 14 is a diagram illustrating that the configuration of sixdifferent physical resource block (PRB) allocations for the serving MRSset can be different for different access nodes and matched to differentaccess node IDs, according to some embodiments.

FIG. 15 is a block diagram illustrating a functional implementation of anetwork node, according to some embodiments.

FIG. 16 is a block diagram illustrating a functional implementation of awireless device, according to some embodiments.

DETAILED DESCRIPTION

An example system may include a UE and a network radio access node,where a wireless device, i.e., a UE, performs RLM in a system withbeamforming by performing RRM measurements based on the same periodicreference signals configured to support connected mode mobility (MRSs).

In the context of the present disclosure, “performing RLM” meansperforming RRM measurements and comparing the value of a given metric,e.g., a signal-to-interference-plus-noise ratio (SINR), with a thresholdthat represents the downlink control channel quality under theassumption that the control channel would have been transmitted in thesame manner, i.e., with similar beamforming properties and/or similar orrepresentative frequency resources.

The measurements over the RSs used for RLM should be correlated with thequality of the downlink (DL) control channel (e.g., PDCCH or ePDCCH inLTE) from which the network is supposed to contact the UE (e.g., bysending scheduling information), despite the fact that different RSs canbe used to estimate the downlink control channel and decode the controlinformation. For example, the UE can use the same MRS to perform RLMwhile the PDCCH decoding is done using UE-specific demodulation RSs(DMRSs). One aspect of this system is that the network guarantees thecorrelation of the quality of the serving cell MRS(s) and the quality ofthe downlink control channel(s). This can be done at the network side bybeamforming the downlink control channel information with the samebeamforming configuration (e.g., direction, beam width, powerdistribution, same antenna panel, etc.) used for transmitting the MRSsconfigured for that UE. Note that as used herein, the terms “MRS” and“mobility reference signal” are used to refer to reference signalsconfigured to and/or used to support connected mode mobility, i.e., formeasurement by UEs to determine when handovers to other beams and/orcells. It will be appreciated that some or all of these referencesignals may be used for other purposes as well, and these referencesignals may be known by other names.

For the MRSs transmitted in one or several beams, different embodimentscan define the information the signal carries, e.g., in terms of theidentifiers, in various ways. In some embodiments, for example,different RSs are transmitted in each beam, and each one carries its ownbeam identifier (BID). In this case, the reference signals can be calledbeam-specific RS (BRS), and the UE can perform RLM on a per-beam basis,i.e., measuring a quality metric, e.g., an RSRP or SINR per individualbeam that is equivalent to the quality of the transmission of thedownlink control channel in that specific beam. In other embodiments,the same RSs may be transmitted in each of the beams, where each onecarries the same identifier. This identifier can either be a BID, agroup identifier that can be a cell identifier Cell ID (CID) or both abeam ID+cell ID. In these embodiments, the UE may distinguish beams inthe time domain, and/or simply perform some averaging over beams 2 scarrying the same identifier.

FIG. 6 illustrates the principles of a reference signal transmissionthat facilitate the RLM procedures described herein. As seen on theleft-hand side of FIG. 6, each beam carries RSs that are configured tothe wireless device (e.g., UE) for mobility purposes. These referencesignals are referred to as mobility reference signals or MRSs herein,though they may not necessarily carry that name. What is meant by“configured to the UE” is that a UE in RRC_CONNECTED mode is providedwith information regarding measurements and reporting conditions, withrespect to serving cell/beam signals and/or non-serving cell/beamsignals. These RSs may carry a BID, a beam ID plus a group ID (which maybe understood as a cell ID, for example), or simply a group ID, invarious embodiments. As seen on the right-hand side of FIG. 6, adownlink control channel, e.g., a PDCCH, is transmitted using the samebeamforming properties as the RSs that are used for mobility purposes.This may be understood as transmitting the downlink control channel inthe “same beam” as the RSs, even if transmitted at different times. Notethat the downlink control channel can carry (or be associated with)different RSs for channel estimation and channel decoding purposes.These can be, but are not necessarily, completely separate from the onesused for mobility, and may be cell-specific, UE-specific, and/orbeam-specific, in various embodiments.

Given the approach shown in FIG. 6, it will be understood that RLM canbe carried out on the MRSs, i.e., the RSs RS-1 to RS-N, since becausethe downlink control channel is beamformed in the same way as the MRSs,the measured quality of the MRSs will directly correspond to a qualityof the downlink control channel. Thus, thresholds for in-sync andout-of-sync detection can be utilized in the same way as in LTE.

However, in order to fulfill requirements for RRM measurements, theseMRSs have been envisioned to be narrow band signal (e.g., 6 centralphysical resource blocks (PRBs)). On the other hand, the downlinkcontrol channel can either be transmitted in the whole band (as LTEPDCCH) or localized/distributed (as LTE ePDCCH and the downlink controlchannel design in NR).

In the case of localized downlink control channels, i.e., where thecontrol channels are transmitted within a relatively small bandwidth,compared to the available bandwidth, such that frequency selectivity ofthe radio channel is insignificant, the system may transmit MRSs in somerepresentative physical resource blocks (PRBs) whose quality iscorrelated with the quality of the PRBs where the downlink controlchannel is transmitted for the UE. However, in the case ofnon-localized/distributed downlink control channels, i.e., where thecontrol channels are transmitted using resource elements that are spreadover the available bandwidth, so as to exploit frequency diversity, thattechnique may provide some inaccuracies in the sense that while the MRSbandwidth is confined to a limited number of PRBs, the downlink controlchannel frequency of the UE may extend to much wider bandwidths so thatthere might be a limited accuracy of the downlink control channelquality estimation based on the MRSs.

Embodiments of the present invention provide a technique where the UEperforms RLM in a system with beamforming by performing RRM measurementsbased on a new signal that is a version of the same periodic referencesignals configured to support connected mode mobility (MRSs), butrepeated in the frequency domain in the same frequency resources as thedownlink control channel of a given UE would be transmitted. Thesemultiple versions of the mobility RSs may also be transmitted indifferent subframes in order to provide some additional time domaindiversity and/or to enable the beamforming transmission to beequivalent.

For example, a method includes performing RLM based on multiple replicasof MRSs, but repeated over multiple frequency resources equivalent tothe frequency resources where the UE's downlink control channels wouldbe transmitted (instead of a single set of resource blocks). At thenetwork side, the radio access node transmits downlink control channelinformation in the same way it transmits the reference signals to bereused for RLM purposes.

In the following, concepts in accordance with exemplary embodiments ofthe invention will be explained in more detail and with reference to theaccompanying drawings. The illustrated embodiments relate to radio linkmonitoring in such a wireless communication network, as performed bywireless devices, in the following also referred to as UEs, and accessnodes. The wireless communication network may for example be based on a5G radio access technology (RAT), such as an evolution of the LTE RAT orthe 3GPP New Radio (NR). However, it is to be understood that theillustrated concepts could also be applied to other RATs.

FIG. 7 illustrates a diagram of a network node 30 that may be configuredto carry out one or more of the disclosed techniques. The network node30 can be any kind of network node that may include a network accessnode such as a base station, radio base station, base transceiverstation, evolved Node B (eNodeB), Node B, gNodeB, or relay node. In thenon-limiting embodiments described below, the network node 30 will bedescribed as being configured to operate as a cellular network accessnode in an NR network.

Those skilled in the art will readily appreciate how each type of nodemay be adapted to carry out one or more of the methods and signalingprocesses described herein, e.g., through the modification of and/oraddition of appropriate program instructions for execution by processingcircuits 32.

The network node 30 facilitates communication between wirelessterminals, other network access nodes and/or the core network. Thenetwork node 30 may include a communication interface circuit 38 thatincludes circuitry for communicating with other nodes in the corenetwork, radio nodes, and/or other types of nodes in the network for thepurposes of providing data and/or cellular communication services. Thenetwork node 30 communicates with UEs using antennas 34 and atransceiver circuit 36. The transceiver circuit 36 may includetransmitter circuits, receiver circuits, and associated control circuitsthat are collectively configured to transmit and receive signalsaccording to a radio access technology, for the purposes of providingcellular communication services.

The network node 30 also includes one or more processing circuits 32that are operatively associated with the transceiver circuit 36 and, insome cases, the communication interface circuit 38. For ease ofdiscussion, the one or more processing circuits 32 are referred tohereafter as “the processing circuit 32” or “the processing circuitry32.” The processing circuit 32 comprises one or more digital processors42, e.g., one or more microprocessors, microcontrollers, Digital SignalProcessors (DSPs), Field Programmable Gate Arrays (FPGAs), ComplexProgrammable Logic Devices (CPLDs), Application Specific IntegratedCircuits (ASICs), or any mix thereof. More generally, the processingcircuit 32 may comprise fixed circuitry, or programmable circuitry thatis specially configured via the execution of program instructionsimplementing the functionality taught herein, or may comprise some mixof fixed and programmed circuitry. The processor 42 may be multi-core,i.e., having two or more processor cores utilized for enhancedperformance, reduced power consumption, and more efficient simultaneousprocessing of multiple tasks.

The processing circuit 32 also includes a memory 44. The memory 44, insome embodiments, stores one or more computer programs 46 and,optionally, configuration data 48. The memory 44 provides non-transitorystorage for the computer program 46 and it may comprise one or moretypes of computer-readable media, such as disk storage, solid-statememory storage, or any mix thereof. Here, “non-transitory” meanspermanent, semi-permanent, or at least temporarily persistent storageand encompasses both long-term storage in non-volatile memory andstorage in working memory, e.g., for program execution. By way ofnon-limiting example, the memory 44 comprises any one or more of SRAM,DRAM, EEPROM, and FLASH memory, which may be in the processing circuit32 and/or separate from the processing circuit 32. In general, thememory 44 comprises one or more types of computer-readable storage mediaproviding non-transitory storage of the computer program 46 and anyconfiguration data 48 used by the network access node 30. The processingcircuit 32 may be configured, e.g., through the use of appropriateprogram code stored in memory 44, to carry out one or more of themethods and/or signaling processes detailed hereinafter.

The network node 30 is configured, according to some embodiments, tooperate as an access node of a wireless communications system thatprovides for a UE to measure its serving cell quality where the cell istransmitting signals in a beamforming manner. The processing circuit 32is configured to transmit, in a first downlink signal having a series ofsubframes, a beam-formed reference signal in each of a plurality ofsubframes, where the beam-formed reference signals are transmitted infewer than all of the subframes of the downlink signal. The beam-formedreference signals include a first subset and an at least partlydiffering second subset, where the first subset includes beam-formedreference signals corresponding to a first frequency or first localizedrange of frequencies, and the second subset includes beam-formedreference signals corresponding to a second frequency or secondlocalized range of frequencies. By “localized range of frequencies” ismeant that the range of frequencies is only a relatively small portionof available bandwidth, such that there is insignificant frequencyselectivity in the radio channel across the range of frequencies. Thesecond frequency or second localized range of frequencies is spacedapart from and differing from the first frequency or first localizedrange of frequencies. The processing circuit 32 is configured toconfigure a UE to perform mobility management measurements using atleast the first subset of the beam-formed reference signals and toperform RLM using at least the second subset of the beam-formedreference signals. In some embodiments, processing circuit 32 isconfigured to transmit a first control channel using the samebeamforming parameters used to transmit the beam-formed referencesignals.

Regardless of the physical implementation, the processing circuit 32 isconfigured to perform, according to some embodiments, a method 800 in anaccess node of a wireless communications system, as shown in FIG. 8. Themethod 800 includes transmitting, in a first downlink signal having aseries of subframes carrying, a beam-formed reference signal in each ofa plurality of subframes, where the beam-formed reference signals aretransmitted in fewer than all of the subframes of the downlink signal(block 804). The beam-formed reference signals include a first subsetand an at least partly differing second subset, the first subsetincludes beam-formed reference signals corresponding to a firstfrequency or first localized range of frequencies, and the second subsetincludes beam-formed reference signals corresponding to a secondfrequency or second localized range of frequencies. The second frequencyor second localized range of frequencies is spaced apart from anddiffering from the first frequency or first localized range offrequencies. The method also includes configuring a UE to performmobility management measurements using at least a first subset of thebeam-formed reference signals and to perform RLM using at least thesecond subset of the beam-formed reference signals (block 802). Theconfiguring may be performed prior to the transmitting, and thetransmitting may include transmitting a first control channel using thesame beamforming parameters used to transmit the beam-formed referencesignals.

The beam-formed reference signals corresponding to the second frequencyor second localized range of frequencies may have a periodicity in timethat differs from a periodicity in time for the beam-formed referencesignals corresponding to the first frequency or first localized range offrequencies. The second subset may further include beam-formed referencesignals corresponding to a third frequency or third localized range offrequencies, the third frequency or third localized range of frequenciesbeing spaced apart from and differing from the first and secondfrequencies or first and second localized range of frequencies.

In some cases, the beam-formed reference signals corresponding to thefirst frequency or first localized range of frequencies may eachcoincide in time with the beam-formed reference signals corresponding tothe second frequency or second localized range of frequencies.

The method 800 may include transmitting one or more additional referencesignals for use by the UE in estimating a channel for the first controlchannel and/or transmitting the first control channel in frequencyresources at least partly overlapping frequency resources carrying thebeam-formed reference signals. The beam-formed reference signals mayinclude a beam-specific reference signal for a first beam. Thebeam-specific reference signal may carry a beam identifier, and themethod 800 may include decoding the beam identifier from thebeam-specific reference signal.

Another aspect of some embodiments is that the beam-formed referencesignals are transmitted periodically and sparse in time, i.e., not inall subframes. However, the periodicity required for RLM may differ fromthe periodicity required for RRM measurements to trigger measurementsreports. Therefore, in some embodiments the UE may only select somespecific samples out of the transmitted RSs for RLM, where thesesample/subframes are possibly configured by the network.

In some cases, for example, the UE is configured with a periodicity ofbeam-formed reference signals, and based on a pre-defined RLMperiodicity in the standards, it performs the RRM measurements for RLM.In other cases, the UE is informed of both periodicities, i.e., oneperiodicity where signals are transmitted and a periodicity to be usedfor RLM matching its discontinuous reception (DRX) cycle.

The method 800 may include transmitting, to the UE, one or more firstconfiguration parameters defining a periodicity and/or frequencylocation for the first subset of beam-formed reference signals. Thismethod 800 may further include transmitting, to the UE, one or moresecond configuration parameters defining a periodicity and/or frequencylocation for the second subset of beam-formed reference signals.

FIG. 9 illustrates a diagram of the corresponding UE, shown as wirelessdevice 50. The wireless device 50 may be considered to represent anywireless terminals that may operate in a network, such as a UE in acellular network. Other examples may include a communication device,target device, device to device (D2D) UE, machine type UE or UE capableof machine to machine communication (M2M), a sensor equipped with UE,PDA (personal digital assistant), Tablet, mobile terminal, smart phone,laptop embedded equipped (LEE), laptop mounted equipment (LME), USBdongles, Customer Premises Equipment (CPE), etc.

The wireless device 50 is configured to communicate with a radio node orbase station in a cellular network via antennas 54 and a transceivercircuit 56. The transceiver circuit 56 may include transmitter circuits,receiver circuits, and associated control circuits that are collectivelyconfigured to transmit and receive signals according to a radio accesstechnology, for the purposes of using cellular communication services.This radio access technology is NR for the purposes of this discussion.

The wireless device 50 also includes one or more processing circuits 52that are operatively associated with the radio transceiver circuit 56.The processing circuit 52 comprises one or more digital processingcircuits, e.g., one or more microprocessors, microcontrollers, DSPs,FPGAs, CPLDs, ASICs, or any mix thereof. More generally, the processingcircuit 52 may comprise fixed circuitry, or programmable circuitry thatis specially adapted via the execution of program instructionsimplementing the functionality taught herein, or may comprise some mixof fixed and programmed circuitry. The processing circuit 52 may bemulti-core.

The processing circuit 52 also includes a memory 64. The memory 64, insome embodiments, stores one or more computer programs 66 and,optionally, configuration data 68. The memory 64 provides non-transitorystorage for the computer program 66 and it may comprise one or moretypes of computer-readable media, such as disk storage, solid-statememory storage, or any mix thereof. By way of non-limiting example, thememory 64 comprises any one or more of SRAM, DRAM, EEPROM, and FLASHmemory, which may be in the processing circuit 52 and/or separate fromprocessing circuit 52. In general, the memory 64 comprises one or moretypes of computer-readable storage media providing non-transitorystorage of the computer program 66 and any configuration data 68 used bythe user equipment 50. The processing circuit 52 may be configured,e.g., through the use of appropriate program code stored in memory 64,to carry out one or more of the methods and/or signaling processesdetailed hereinafter.

The wireless device 50 is configured, according to some embodiments, tomeasure a serving cell quality where the cell is transmitting signals ina beamforming manner. Accordingly, the processing circuit 52 isconfigured to receive, in a downlink signal having a series ofsubframes, a beam-formed reference signal in each of a plurality ofsubframes, where the beam-formed reference signals are received in fewerthan all of the subframes of the downlink signal. The processing circuit52 is also configured to perform mobility management measurements usingat least a first subset of the received beam-formed reference signals,the first subset corresponding to a first frequency or first localizedrange of frequencies. The processing circuit 52 is also configured toperform RLM using a second subset of the received beam-formed referencesignals, the second subset at least partly differing from the firstsubset and including beam-formed reference signals corresponding to asecond frequency or second localized range of frequencies. The secondfrequency or second localized range of frequencies is spaced apart fromand differing from the first frequency or first localized range offrequencies.

According to some embodiments, the processing circuit 52 is configuredto perform a corresponding method 1000, shown in FIG. 10, that includesreceiving, in a downlink signal having a series of subframes, abeam-formed reference signal in each of a plurality of subframes, wherethe beam-formed reference signals are received in fewer than all of thesubframes of the downlink signal (block 1002). The method 1000 alsoincludes performing mobility management measurements using at least afirst subset of the received beam-formed reference signals, the firstsubset corresponding to a first frequency or first localized range offrequencies (block 1004). The method 1000 further includes performingRLM using a second subset of the received beam-formed reference signals,the second subset at least partly differing from the first subset andincluding beam-formed reference signals corresponding to a secondfrequency or second localized range of frequencies (block 1006). Thesecond frequency or second localized range of frequencies is spacedapart from and differing from the first frequency or first localizedrange of frequencies.

In some cases, the beam-formed reference signals correspond to thesecond frequency or second localized range of frequencies have aperiodicity in time that differs from a periodicity in time forbeam-formed reference signals corresponding to the first frequency orfirst localized range of frequencies. The second subset may includebeam-formed reference signals corresponding to a third frequency orthird localized range of frequencies, the third frequency or thirdlocalized range of frequencies being spaced apart from and differingfrom the first and second frequencies or first and second localizedrange of frequencies. The beam-formed reference signals may correspondto the first frequency or first localized range of frequencies eachcoincide in time with the beam-formed reference signals corresponding tothe second frequency or second localized range of frequencies.

Performing RLM may include performing one or more measurements using theat least some of the same beam-formed reference signals to obtain ametric, and comparing the metric to a threshold that represents apredetermined downlink control channel quality, given an assumption thata hypothetical control channel corresponding to the control channelquality is transmitted using the same beamforming properties applied tothe beam-formed reference signals. The method 1000 may includedemodulating a first control channel using one or more additionalreference signals to estimate a channel for the first control channel.The first control channel may be received in frequency resources atleast partly overlapping frequency resources carrying the beam-formedreference signals used for performing RLM.

Performing RLM may also include determining that the UE is in-sync orout-of-sync, based on measurements of the at least some of the samebeam-formed reference signals.

In some cases, the at least some of the same beam-formed referencesignals comprise a beam-specific reference signal for a first beam, andperforming RLM may include performing RLM for the first beam, using thebeam-specific reference signal. The beam-specific reference signal maycarry a beam identifier, and the method 1000 may include decoding thebeam identifier from the beam-specific reference signal.

The method 1000 may also include receiving, prior to performing saidmobility management measurements, one or more first configurationparameters defining a periodicity and/or frequency location for thefirst subset of beam-formed reference signals. The method 1000 mayfurther include receiving, prior to performing said RLM, one or moresecond configuration parameters defining a periodicity and/or frequencylocation for the second subset of beam-formed reference signals.

The problem and techniques describing the solution will be furtherexplained. As seen in the example configuration shown in FIG. 11, thetransmission of the RSs used for mobility can be configured sparsely forRRM and synchronization functions, in the time and frequency domains.For example, the RSs used for mobility can be transmitted on sixadjacent PRBs in every fifth subframe, as illustrated in FIG. 11.

However, such a time-frequency resource granularity of MRSs in theserving MRS set is not as abundant as the PDCCH occasions on theresource grid. The number of measurement samples during the RLMprocedure should be sufficiently large to capture the quality of thetime/frequency resources where the downlink control channel istransmitted. Therefore, the samples should be taken on many subcarriersthroughout the downlink transmission bandwidth. The frequency allocationof serving MRSs used for RLM can be based on a localized or adistributed scheme for the downlink control channels. A localized schememay require fewer UE computations, whereas a distributed scheme mayprovide better accuracy in frequency-selective channels.

As suggested by the example shown in FIG. 11, to fulfill requirementsfor RRM measurements, the mobility RSs have been envisioned to be anarrow band signal, e.g., occupying only six central PRBs. On the otherhand, the downlink control channel can either be transmitted in thewhole band (as in the case of the LTE PDCCH) or localized/distributed(as in the case of LTE ePDCCH).

In the case of localized downlink control channels, the MRSs may betransmitted in some representative PRBs whose quality is correlated withthe quality of the PRBs where the downlink control of the UE would betransmitted. However, in the case of a non-localized/distributeddownlink control channel, this technique may provide some inaccuraciesin the sense that while the MRS bandwidth is confined to a limitednumber of PRBs, the frequency allocation of the downlink control channelmay extend to much wider bandwidths, such that there might be a limitedaccuracy of the downlink control channel quality estimation based on therelatively narrowband mobility RSs.

Embodiments of the techniques and apparatus disclosed herein addressthis problem, and include a method at a UE and a network radio accessnode where the UE performs RLM in a system with beamforming byperforming RRM measurements based on a new signal that is a version ofthe same periodic reference signals configured to support connected modemobility (MRSs), but repeated in the frequency domain in the frequencyresources where the downlink control channel of a given UE would betransmitted. These multiple versions of the MRSs may also be transmittedin different subframes, to provide some additional time domain diversityand/or to enable the beamforming transmission to be equivalent.

One advantage of this approach is that instead of creating a largeoverhead by transmitting MRSs over much more frequency resources, thisapproach leverages the fact that the RLM periodicity requirements islarger than the mobility requirements, allowing for sparser referencesignals to be used for RLM. Thus, the replicated versions of the MRS aretransmitted more sparsely in the time and frequency domains than theMRSs, reducing the overhead and/or the static interference caused by theRSs. Another advantage is that the replicated RSs used only for RLMpurposes can be switched off once there are no active UE's in the cell.Overall, this approach ensures that UEs can take more accurate RLMmeasurements over a large range of time-frequency resources, withoutintroducing a static/always-on periodic RS in the network.

Other advantages include that the signaling overhead is maintained at alow level, without compromising the accuracy of RLM measurements,especially during data inactivity. This is expected to be an importantrequirement in 5G NR. Further, these techniques provide accurate RLMalso when the downlink control channel frequency allocation is extendedover a larger bandwidth than that defined for MRSs.

According to the presently disclosed techniques, then, the wirelessdevice (e.g., UE) performs RRM measurements for RLM using RSsdistributed over multiple, spaced apart, frequency resources, instead ofusing only a single localized set of resource blocks. To support this,the UE in some embodiments is provided with two types of configurationsfor RSs of the same type. This may be done, e.g., using the RadioResource Configuration (RRC) protocol, e.g., via a RRC ConnectionRe-Configuration message. First, the UE is provided with a mobilityconfiguration, which specifies the frequency resources, such as PRBs inwhich the MRSs are transmitted with periodicity T_mobility, as well asthe time-domain resources, e.g., the subframes, in which these aretransmitted. The UE can then measure the MRSs in these resources asneeded for mobility purposes. Second, the UE is provided with an RLMconfiguration, which specifies the additional frequency resources, e.g.,PRBs, in which the MRSs are transmitted with periodicity T_RLM, as wellas the time-domain resources, e.g., subframes, in which these additionalMRSs are transmitted. The UE can then use any or all of the MRSs inthese additional time-frequency resources (as well as those specified inthe mobility configuration) for RLM purposes.

Note that in some embodiments there may be a sub-configuration wherethese additional MRSs are transmitted in the same subframes (or anyother time resource indicated) as the ones used for mobility, butpossibly with different periodicity.

FIG. 12 illustrates an example of how the MRSs may be transmitted, tosupport both mobility measurements and RLM. In the illustrated example,MRSs are transmitted in frequency resources localized at F1, at arelatively frequency periodicity, e.g., 5 milliseconds, for mobilitymeasurement purposes. The UE may be configured with configurationinformation specifying these time-frequency resources, e.g., with aparameter specifying F1, a parameter indicating a 5 millisecondsperiodicity, etc., and then use the RSs transmitted in thesetime-frequency resources for mobility measurements. Note that F1, F2,F3, etc., may indicate a set or range of subcarriers in someembodiments. For example, the MRSs may occupy six adjacent PRBs at eachof the locations in the frequency band indicated by F1, F2, and F3 inthe figure. Configuration parameters provided to the UE, e.g., by RRCsignaling, may indicate a center frequency, lower frequency, or someother pointer to a frequency position or range, and may, in someembodiments, even indicate a bandwidth across which a localized group ofRSs are transmitted.

In FIG. 12, the RSs at F1 are provided for mobility measurementpurposes, and have a periodicity sufficient for these purposes. Theexample configuration shown in FIG. 12 also includes additional RSs, ofthe same type, but at different frequencies F2 and F3, and with adifferent periodicity. The extended periodicity, here shown as fourtimes the period of the RS periodicity for mobility purposes, reflectsthe fact that RLM requires less frequent measurements. Placing these RSsat different frequencies, however, allows for the RLM to be moreaccurately correlated with downlink control channel transmissions, e.g.,in the case where the downlink control channel or control channel searchspace is distributed across the frequency band.

Note that while it may be convenient in some embodiments for theperiodicity of the additional RSs to be an integer multiple of the RSsused for mobility purposes, this is not necessarily the case. Also,while the additional RSs at F2 and F3 in FIG. 12 are shown as coincidingin time with some of the RSs at F1, this again is not necessarily thecase—these may be offset in time, in some embodiments. This is the casewith the example configuration shown in FIG. 13. Further, theseadditional RSs need not even coincide in time with one another—this isalso shown in FIG. 13, where the additional RSs at F2 and F3 are offsetfrom one another by two subframes. Still further, these additional RSsneed not even have the same periodicity, at different frequencies. Thus,for example, the RSs at F2 may have a different periodicity than thoseat F3.

One aspect of the techniques described above is that the networktransmits the RSs to be used for RLM in frequency resources that arecorrelated (i.e., overlapping or closely corresponding in frequency)with those where the downlink control channel is being transmitted.Thus, if the RSs are transmitted using the same beamforming propertiesas those applied to the downlink control channel, the result is that theRS quality is both correlated in the directional domain (which might bereferred to as “the beam domain”) and in the frequency domain,regardless of any further time averaging that may occur.

The transmission of the RSs used for mobility can be configured sparselyfor RRM and synchronization functions in the time and frequency domains.For example, the RSs used for mobility can be transmitted on sixadjacent PRBs in every fifth subframe, as illustrated in FIG. 14.

For the mobility RSs transmitted in one or several beams, differentembodiments can define the information for the signal carriers, forexample, in terms of the identifiers.

In one case, different RSs are transmitted in the beams and each onecarries its own beam identifier (ID). They can then be calledbeam-specific RS (BRS) and the UE could perform RLM per beam. That is,the UE can measure a metric, e.g., the reference signal received power(RSRP) or the signal-to-interference-plus-noise ratio (SINR) perindividual beam equivalent to the quality of the transmission of thedownlink control channel in that specific beam.

In a second case, the same RSs are transmitted in the beams and each onecarries the same identifier, which can either be a beam identifier(BID), a group identifier that can be a cell identifier Cell ID (CID) orboth a beam ID+cell ID. In this case, the UE distinguishes beams in thetime domain and/or simply performs some averaging over beams carryingthe same identifier.

In one aspect, the network transmits these RSs to be used for RLM incorrelated frequency resources where the downlink control channel isbeing transmitted so that the RS quality is correlated in the frequencydomain despite further time averaging that may occur. If the downlinkcontrol channel is transmitted in the same beam(s) as the RSs used forRLM, the RS quality is correlated in the directional domain (beamdomain) as well.

In another aspect, because the RSs used for mobility has itstransmission periodic and sparse in time (i.e. not in all subframes),the periodicity required for RLM may differ from the periodicityrequired for RRM measurements to trigger measurements reports.Therefore, the UE may only select some specific samples out of thetransmitted RSs where these sample/subframes are possibly configured bythe network. Alternatively, the periodicity of the RS used for RLM maybe shorter than the periodicity of the RS used for mobility.

The techniques described herein provide a configurable and dynamicmethod to perform reference signal measurements for the RLM function atUEs, without violating the lean signaling principles of 3GPP 5G NR. Animportant advantage enabled by these techniques is an improvedefficiency at which the network can flexibly configure a limited numberof sparse reference signals for different deployment (e.g., number ofbeams) and traffic (e.g., number of users, data activity/inactivity)scenarios.

As discussed in detail above, the techniques described herein, e.g., asillustrated in the process flow diagrams of FIGS. 8 and 10, may beimplemented, in whole or in part, using computer program instructionsexecuted by one or more processors. It will be appreciated that afunctional implementation of these techniques may be represented interms of functional modules, where each functional module corresponds toa functional unit of software executing in an appropriate processor orto a functional digital hardware circuit, or some combination of both.

FIG. 15 illustrates an example functional module or circuit architectureas may be implemented in an access node of a wireless communicationnetwork, such as in network node 30. The functional implementationincludes a transmitting module 1504 for transmitting, in a downlinksignal having a series of subframes, a beam-formed reference signal ineach of a plurality of subframes, where the beam-formed referencesignals are transmitted in fewer than all of the subframes of thedownlink signal, and where the beam-formed reference signals include afirst subset and an at least partly differing second subset. The firstsubset includes beam-formed reference signals corresponding to a firstfrequency or first localized range of frequencies, and the second subsetincludes beam-formed reference signals corresponding to a secondfrequency or second localized range of frequencies. The second frequencyor second localized range of frequencies being spaced apart from anddiffering from the first frequency or first localized range offrequencies. The implementation also includes a configuring module 1502for configuring a UE to perform mobility management measurements usingat least the first subset of the beam-formed reference signals and toperform RLM using at least the second subset of the beam-formedreference signals. The transmitting module 1504 is also for transmittinga first control channel using the same beamforming parameters used totransmit the beam-formed reference signals.

FIG. 16 illustrates an example functional module or circuit architectureas may be implemented in a wireless device 50 adapted for operation in awireless communication network. The implementation includes a receivingmodule 1602 for receiving, in a downlink signal having a series ofsubframes, a beam-formed reference signal in each of a plurality ofsubframes, where the beam-formed reference signals are received in fewerthan all of the subframes of the downlink signal. The implementationalso includes a mobility management module 1604 for performing mobilitymanagement measurements using at least a first subset of the receivedbeam-formed reference signals, the first subset corresponding to a firstfrequency or first localized range of frequencies and a radio linkmonitoring module 1606 for performing RLM using a second subset of thereceived beam-formed reference signals. The second subset at leastpartly differs from the first subset and includes beam-formed referencesignals corresponding to a second frequency or second localized range offrequencies. The second frequency or second localized range offrequencies is spaced apart from and differing from the first frequencyor first localized range of frequencies.

1-50. (canceled)
 51. A method, in a user equipment (UE) comprising:receiving, in a downlink signal having a series of subframes, abeam-formed reference signal in each of a plurality of subframes,wherein the beam-formed reference signals are received in fewer than allof the subframes of the downlink signal; performing mobility managementmeasurements using at least a first subset of the received beam-formedreference signals, the first subset corresponding to a first frequencyor first localized range of frequencies; and performing radio linkmonitoring (RLM) using a second subset of the received beam-formedreference signals, the second subset at least partly differing from thefirst subset and including beam-formed reference signals correspondingto a second frequency or second localized range of frequencies, thesecond frequency or second localized range of frequencies being spacedapart from and differing from the first frequency or first localizedrange of frequencies.
 52. A method, in an access node of a wirelesscommunications system, the method comprising: transmitting, in adownlink signal having a series of subframes, a beam-formed referencesignal in each of a plurality of subframes, wherein the beam-formedreference signals are transmitted in fewer than all of the subframes ofthe downlink signal, and wherein the beam-formed reference signalsinclude a first subset and an at least partly differing second subset,the first subset including beam-formed reference signals correspondingto a first frequency or first localized range of frequencies, and thesecond subset including beam-formed reference signals corresponding to asecond frequency or second localized range of frequencies, the secondfrequency or second localized range of frequencies being spaced apartfrom and differing from the first frequency or first localized range offrequencies; and configuring a user equipment (UE) to perform mobilitymanagement measurements using at least the first subset of thebeam-formed reference signals and to perform radio link monitoring (RLM)using at least the second subset of the beam-formed reference signals.53. A user equipment (UE) comprising: transceiver circuitry; andprocessing circuitry operatively associated with the transceivercircuitry and configured to: receive, using the transceiver circuitry,in a downlink signal having a series of subframes, a beam-formedreference signal in each of a plurality of subframes, wherein thebeam-formed reference signals are received in fewer than all of thesubframes of the downlink signal; perform mobility managementmeasurements using at least a first subset of the received beam-formedreference signals, the first subset corresponding to a first frequencyor first localized range of frequencies; and perform radio linkmonitoring (RLM) using a second subset of the received beam-formedreference signals, the second subset at least partly differing from thefirst subset and including beam-formed reference signals correspondingto a second frequency or second localized range of frequencies, thesecond frequency or second localized range of frequencies being spacedapart from and differing from the first frequency or first localizedrange of frequencies.
 54. The UE of claim 53, wherein the beam-formedreference signals corresponding to the second frequency or secondlocalized range of frequencies have a periodicity in time that differsfrom a periodicity in time for the beam-formed reference signalscorresponding to the first frequency or first localized range offrequencies.
 55. The UE of claim 53, wherein the second subset furtherincludes beam-formed reference signals corresponding to a thirdfrequency or third localized range of frequencies, the third frequencyor third localized range of frequencies being spaced apart from anddiffering from the first and second frequencies or first and secondlocalized range of frequencies.
 56. The UE of claim 53, wherein thebeam-formed reference signals corresponding to the first frequency orfirst localized range of frequencies each coincide in time withbeam-formed reference signals corresponding to the second frequency orsecond localized range of frequencies.
 57. The UE of claim 53, whereinthe processing circuitry is configured to perform RLM by performing oneor more measurements using the at least some of the same beam-formedreference signals to obtain a metric, and compare the metric to athreshold that represents a predetermined downlink control channelquality.
 58. The UE of claim 57, wherein the processing circuitry isconfigured to demodulate a first control channel using one or moreadditional reference signals to estimate a channel for the first controlchannel.
 59. The UE of claim 58, wherein the first control channel isreceived in frequency resources at least partly overlapping frequencyresources carrying the beam-formed reference signals used for performingRLM.
 60. The UE of claim 53, wherein the processing circuitry isconfigured to perform RLM by determining that the UE is in-sync orout-of-sync, based on measurements of the at least some of the samebeam-formed reference signals.
 61. The UE of claim 53, wherein the atleast some of the same beam-formed reference signals comprise abeam-specific reference signal for a first beam, and wherein theprocessing circuitry is configured to perform RLM by performing RLM forthe first beam, using the beam-specific reference signal.
 62. The UE ofclaim 61, wherein the beam-specific reference signal carries a beamidentifier, and wherein the processing circuitry is configured to decodethe beam identifier from the beam-specific reference signal.
 63. The UEof claim 53, wherein the processing circuitry is configured to receive,prior to performing said mobility management measurements, one or morefirst configuration parameters defining a periodicity and/or frequencylocation for the first subset of beam-formed reference signals.
 64. TheUE of claim 63, wherein the processing circuitry is configured toreceive, prior to performing said RLM, one or more second configurationparameters defining a periodicity and/or frequency location for thesecond subset of beam-formed reference signals.
 65. An access node of awireless communications system, comprising: transceiver circuitry; andprocessing circuitry operatively associated with the transceivercircuitry and configured to: transmit, using the transceiver circuitry,in a downlink signal having a series of subframes, a beam-formedreference signal in each of a plurality of subframes, wherein thebeam-formed reference signals are transmitted in fewer than all of thesubframes of the downlink signal, and wherein the beam-formed referencesignals include a first subset and an at least partly differing secondsubset, the first subset including beam-formed reference signalscorresponding to a first frequency or first localized range offrequencies, and the second subset including beam-formed referencesignals corresponding to a second frequency or second localized range offrequencies, the second frequency or second localized range offrequencies being spaced apart from and differing from the firstfrequency or first localized range of frequencies; and configure a userequipment (UE) to perform mobility management measurements using atleast the first subset of the beam-formed reference signals and toperform radio link monitoring (RLM) using at least the second subset ofthe beam-formed reference signals.
 66. The access node of claim 65,wherein the beam-formed reference signals corresponding to the secondfrequency or second localized range of frequencies have a periodicity intime that differs from a periodicity in time for the beam-formedreference signals corresponding to the first frequency or firstlocalized range of frequencies.
 67. The access node of claim 65, whereinthe second subset further includes beam-formed reference signalscorresponding to a third frequency or third localized range offrequencies, the third frequency or third localized range of frequenciesbeing spaced apart from and differing from the first and secondfrequencies or first and second localized range of frequencies.
 68. Theaccess node of claim 65, wherein the beam-formed reference signalscorresponding to the first frequency or first localized range offrequencies each coincide in time with the beam-formed reference signalscorresponding to the second frequency or second localized range offrequencies.
 69. The access node of claim 65, wherein the processingcircuitry is configured to transmit, using the transceiver circuitry,one or more additional reference signals for use by the UE in estimatinga channel for a first control channel.
 70. The access node of claim 65,wherein the processing circuitry is configured to transmit, using thetransceiver circuitry, a first control channel using the samebeamforming parameters used to transmit the beam-formed referencesignals, in frequency resources at least partly overlapping frequencyresources carrying the beam-formed reference signals.
 71. The accessnode of claim 65, wherein one or more of the beam-formed referencesignals comprises a beam-specific reference signal for a first beam. 72.The access node of claim 71, wherein the beam-specific reference signalcarries a beam identifier, and wherein the processing circuitry isconfigured to decode the beam identifier from the beam-specificreference signal.
 73. The access node of claim 65, wherein theprocessing circuitry is configured to transmit to the UE, using thetransceiver circuitry, one or more first configuration parametersdefining a periodicity and/or frequency location for the first subset ofbeam-formed reference signals.
 74. The access node of claim 73, whereinthe processing circuitry is configured to transmit to the UE, using thetransceiver circuitry, one or more second configuration parametersdefining a periodicity and/or frequency location for the second subsetof beam-formed reference signals.
 75. A non-transitory computer readablestorage medium storing a computer program comprising programinstructions that, when executed on at least one processing circuit of auser equipment (UE) configured for operation in a wireless communicationnetwork, configures the UE to: receive, in a downlink signal having aseries of subframes, a beam-formed reference signal in each of aplurality of subframes, wherein the beam-formed reference signals arereceived in fewer than all of the subframes of the downlink signal;perform mobility management measurements using at least a first subsetof the received beam-formed reference signals, the first subsetcorresponding to a first frequency or first localized range offrequencies; and perform radio link monitoring (RLM) using a secondsubset of the received beam-formed reference signals, the second subsetat least partly differing from the first subset and includingbeam-formed reference signals corresponding to a second frequency orsecond localized range of frequencies, the second frequency or secondlocalized range of frequencies being spaced apart from and differingfrom the first frequency or first localized range of frequencies.
 76. Anon-transitory computer readable storage medium storing a computerprogram comprising program instructions that, when executed on at leastone processing circuit of an access node of a wireless communicationnetwork, configures the access node to: transmit, in a first downlinksignal having a series of subframes, a beam-formed reference signal ineach of a plurality of subframes, wherein the beam-formed referencesignals are transmitted in fewer than all of the subframes of thedownlink signal, and wherein the beam-formed reference signals include afirst subset and an at least partly differing second subset, the firstsubset including beam-formed reference signals corresponding to a firstfrequency or first localized range of frequencies, and the second subsetincluding beam-formed reference signals corresponding to a secondfrequency or second localized range of frequencies, the second frequencyor second localized range of frequencies being spaced apart from anddiffering from the first frequency or first localized range offrequencies; and configure a user equipment (UE) to perform mobilitymanagement measurements using at least the first subset of thebeam-formed reference signals and to perform radio link monitoring (RLM)using at least the second subset of the beam-formed reference signals.